Overcurrent protective devices required for the tripping of a transformer circuit breaker must have a relatively wide range of response so as to be capable of responding to slight overcurrent conditions and still be capable of withstanding relatively high currents for a short time when major faults occur. For instance, a primary breaker might be called on to interrupt currents as low as 5-10 amperes and still must withstand primary fault currents of 1500-2000 amperes for 10-100 milliseconds. The overcurrent sensor should respond both to the transformer oil ambient temperature and current flow through the device. In my co-pending application Ser. No. 371,776, filed Apr. 26, 1982, and entitled, "Primary Circuit Breaker," which is now U.S. Pat. No. 4,435,690 a primary circuit breaker was disclosed which was connected to the primary winding of the transformer and to interrupt the primary circuit under both fault current and overload conditions. The circuit breaker was provided with a temperature sensing device which responds to an increase in temperature due to a fault current in the primary circuit, as well as an increase in temperature of the insulating oil due to overloading or incipient faults. Although the circuit breaker has performed satisfactorily for its intended purpose, it has been found desirable to obtain a more accurate circuit breaker temperature responsive device to control the response characteristic of the circuit breaker.